A year ago, President Barack Obama used the phrase "red line" twice, to make the point that Syria's use of chemical weapons "would change my calculations significantly" on intervening in the civil war.

Secretary of State John Kerry went before lawmakers in Washington to argue for military intervention. Kerry says President Obama has not told him what he will do if Congress does not vote in favor of a military strike in Syria, but reiterated the president’s assertion that he does not need congressional authorization before initiating action.

“I don’t think the United States – and I think that is the biggest problem about Vietnam - can necessarily apply moral, moralisms to its commitments around the world,” Kerry said on television in 1971.

House Speaker John Boehner (R-Ohio) says he supports the president’s plan for a military strike on Syria and his colleagues “should support this call for action.”